A common problem in hermetic rotary compressors, including those of the scroll type, is the tendency of compressed refrigerant gas to flow back from the discharge pressure portion or so-called high side of the compressor shell, through the compression mechanism and back to the suction pressure portion or so-called low side of the shell upon compressor shutdown. Such backflow is as a result of the natural tendency of the system within which the compressor is employed to equalize its internal pressure when the compressor is de-energized and the fact that scroll compressors, unlike reciprocating compressors, do not rely on the use of a discharge valve against which to pump in order to effect gas compression. Discharge gas backflow, if not prevented, can cause the reverse direction rotation of the scroll members (which comprise the compression mechanism) at speeds far higher than that at which the compressor is intended to operate and can lead to serious compressor damage, such as due to inadequate bearing lubrication, under such circumstances.
The prevention of discharge gas backflow upon shutdown in a scroll compressor is typically accomplished by the disposition of a discharge check valve downstream of the aperture through which compressed gas is discharged from the compressor's compression mechanism. The discharge check valve is carried shut by the initial backflow of refrigerant gas which occurs immediately upon compressor shutdown. The closing of the discharge check valve may be assisted or accelerated by a biasing member such as a spring.
In scroll compressors having compression mechanisms protected from discharge gas-driven reverse rotation by apparatus such as a discharge check valve, a problem can arise when the compressor is electrically connected in an improper manner. Such improper electrical connection, like discharge gas backflow, can cause the compressor motor and, therefore, the compression mechanism, to operate in a direction reverse from that in which they are intended to rotate. This problem is recognized in U.S. Pat. Nos. 4,820,130; 4,840,545; 5,186,613 and 5,290,154 all of which are assigned to the assignee of the present invention.
Briefly, if a scroll compressor having a discharge check valve is miswired so that it is caused to rotate in an unintended direction, the pockets defined between the scroll wraps, instead of moving radially inward and decreasing in volume, move radially outward and expand in volume in a pumping action. In effect, the scroll device functions as a gas expander rather than a compressor under such circumstances.
The expansion of the pockets defined by the scroll members under these circumstances, unless otherwise accommodated for, can cause low and even negative pressures to develop within the pockets defined by the scroll members because the discharge check valve, being closed, gives the mechanism no source of gas to pump from in its motor-driven reverse direction rotation. As a result, the scroll members can be drawn tightly together which will eventually result, to the extent the compressor continues to be motor-driven in the wrong direction, in severe damage to and possibly destruction of the compressor.
Still another difficulty and potential source for damage in scroll compressors is the development of high discharge gas temperatures. Such high discharge temperatures can be caused, among other things, by the operation of the compressor in a system where pressure ratios develop that are outside of the compressor's normal operating range and can result in thermal growth within the compressor, particularly in the wraps of the scroll members. Thermal expansion of the scroll wraps can lead to high wrap tip contact loads and the galling of the wrap tips.
The need continues to exist to efficiently and effectively protect hermetic scroll compressors from the damage which can result from their improper electrical hookup and from the occurrence of high discharge gas temperatures.